1. Field of the Invention
The present invention relates to particle monitoring in a manufacturing process which involves a corrosive or coating agent carried in the process gas. In particular, the present invention relates to particle monitoring in plasma etching or similar processes.
2. Discussion of the Related Art
In a semiconductor fabrication process, plasma etching is often used to selectively remove portions of thin films formed from the surface of a semiconductor wafer. FIG. 1 shows a typical configuration in a process chamber 100 of a plasma etcher. As shown in FIG. 1, a wafer 101 is placed on an electrically grounded platform 102, which also serves as the ground electrode. Placed in close proximity, and substantially parallel to the grounded platform 102, is a "shower head" electrode 103, which applies either a DC (direct current) or an RF (radio frequency) voltage difference across the grounded platform 102 and itself. A plasma, generally represented by reference numeral 104, is created by the applied voltage in the gap above the wafer 101 between grounded platform 102 and "shower head" electrode 103. Process chemicals, such as freon, are carried in an inert gas and injected into the plasma through small holes in shower head electrode 103. These chemicals break down in the plasma, forming reactants that etch material from the surface of wafer 101.
Particles are formed as a byproduct of a plasma etching process. Such particles, when deposited on the surface of wafer 101, results in defects which can reduce the yield of the fabrication process. The control and removal of such particles are therefore critical to reducing the cost of manufacturing. Thus, an optical particle monitor 104 has been developed for use in an environment such as the plasma etching process. An example of an optical particle monitor is disclosed in U.S. Pat. No. 5,132,548, entitled "High Sensitivity Large Detection Area Particle Sensor for Vacuum Applications" to Borden et al, issued Jul. 22, 1992. Optical particle sensor 104 is typically installed in a vacuum exhaust line, such as vacuum exhaust line 105, which carries gas out of the process chamber 100.
Particle sensor 104 is susceptible to the reactants in process chamber 100. In an etching process, reactants such as fluorine compounds and certain polymer byproducts are often created. Fluorine compounds etch and corrode the surfaces of optics in particle sensor 104. In addition, polymer byproducts coat the optic surfaces of particle sensor 104. Such corrosion or coating reduces the performance of optical particle sensor 104. Hence, there is a strong incentive to prevent corrosion or coating on critical optical surfaces of optical particle sensor 104.
In the prior art, various techniques have been used to reduce contamination on the optical surfaces of a particle sensor. One such technique used in an airborne particle sensor is shown in FIG. 2. In FIG. 2, an airborne particle sensor 200 uses filtered air which is combined with a carrier gas stream 204. As shown in FIG. 2, to monitor the particle level in a carrier gas stream 204, carrier gas stream is injected by nozzle 202 through a laser beam 201. Particles carried in carrier gas stream 204 scatter the light in laser beam 201 onto a photodetector 203 mounted in close proximity of laser beam 201. The output signal level of photodetector 203 is a measure of the number of particles in carrier gas stream 204.
In a typical system, laser beam 201 is provided by open cavity laser 205. In open cavity laser 205, one of the mirrors 206 is placed in a position removed from the tube 205a which houses the gas used to generate the laser beam. The gas most often used is a mixture of helium and neon. By placing mirror 206 away from laser tube 205a, open cavity laser 205 creates a portion of the laser beam external to laser tube 205a exposed to the particles to be detected. A problem with using open cavity laser 205 results from particle contamination on the surfaces of the mirror 206 or the window on the laser tube 205a. Particles deposited on these surfaces can scatter enough light from laser beam 201 to make the cavity operate at reduced efficiency. To circumvent this problem, filtered air is injected in the gas line 207 around nozzle 202, so as to provide a jacket of clean air around carrier gas stream 204 carrying the particles. Under this arrangement, particles are prevented from reaching the optics of open cavity laser 205.
However, the technique described above cannot be readily adapted to be used in a low pressure system, since the filtered gas flow must be large, as compared to the flow in carrier gas stream 204. Large filtered gas flow is usually impossible to achieve in the pumping system of the process equipment without seriously affecting the pumping performance. Further, the full diameter of exhaust line 207 of the process equipment is usually required for pumping the process gas, so there is not room remaining for such a shield gas layer.
Thus, it is desirable to provide a means for protecting optics in a particle monitor used in low pressure processes, without seriously affecting the pumping performance of the host processing equipment.